Both mitochondria and chloroplasts can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes encode only some of the proteins of which they are made.

Both mitochondria and chloroplasts have their own genome, and it resembles that of bacteria not that of the nuclear genome.

Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes.

The first amino acid of their transcripts is always fMet as it is in bacteria (not methionine [Met] that is the first amino acid in eukaryotic proteins).

A number of antibiotics (e.g., streptomycin) that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.

Conversely, inhibitors (e.g., diphtheria toxin) of protein synthesis by eukaryotic ribosomes do not — sensibly enough — have any effect on bacterial protein synthesis nor on protein synthesis within mitochondria and chloroplasts.

The antibiotic rifampicin, which inhibits the RNA polymerase of bacteria, also inhibits the RNA polymerase within mitochondria. It has no such effect on the RNA polymerase within the eukaryotic nucleus.

All these gene products are used within the mitochondrion, but the mitochondrion also needs >900 different proteins as well as some mRNAs and tRNAs encoded by nuclear genes. The proteins (e.g., cytochrome c and the DNA polymerases used within the mitochondrion) are synthesized in the cytosol and then imported into the mitochondrion.

genes for 19 of the ~60 proteins used to construct the chloroplast ribosome

All these gene products are used within the chloroplast, but all the chloroplast structures also depend on proteins

encoded by nuclear genes

translated in the cytosol, and

imported into the chloroplast.

RUBISCO, for example, the enzyme that adds CO2 to ribulose bisphosphate to start the Calvin cycle, consists of 8 copies of each of two subunits:

a large one encoded in the chloroplast genome and synthesized within the chloroplast, and

a small subunit encoded in the nuclear genome and synthesized by ribosomes in the cytosol. The small subunit must then be imported into the chloroplast.

The arrangement of genes shown in the figure is found not only in the Bryophytes (mosses and liverworts) but also in the lycopsids (e.g., Lycopodium and Selaginella). In all other plants, however, the portion of DNA bracketed by the red arrows on the left is inverted. The same genes are present but in inverted order. The figure is based on the work of Ohyama, K., et al., Nature, 322:572, 7 Aug 1986; and Linda A. Raubeson and R. K. Jansen, Science, 255:1697, 27 March 1992.

The evolution of the eukaryotic chloroplast by the endosymbiosis of a cyanobacterium in a mitochondria-containing eukaryotic host cell led to the evolution of

Once both heterotrophic and photosynthetic eukaryotes had evolved, the former repeatedly engulfed the latter to exploit their autotrophic way of life. Many animals living today engulf algae for this purpose [Link to examples]. Usually the partners in these mutualistic relationships can be grown separately.

However, a growing body of evidence indicates that the chloroplasts of some algae have not been derived by engulfing cyanobacteria in a primary endosymbiosis like those discussed above, but by engulfing photosynthetic eukaryotes. This is called secondary endosymbiosis. It occurred so long ago that these endosymbionts cannot be cultured away from their host.

In two groups, the eukaryotic nature of the endosymbiont can be seen by its retention of a vestige of a nucleus (called its nucleomorph).

A group of unicellular, motile algae called cryptomonads appear to be the evolutionary outcome of a nonphotosynthetic eukaryotic flagellate (i.e., a protozoan) engulfing a red alga by endocytosis.

Another tiny group of unicellular algae, called chlorarachniophytes, appear to be the outcome of a flagellated protozoan having engulfed a green alga.

The apicoplast (short for "apicomplexan plastid") is a solitary organelle found in the apicomplexan protists: "sporozoans" like Plasmodium falciparum (and the other agents of malaria) and Toxoplasma gondii.

Features:

Essential - the organisms cannot survive without it;

Encased by 4 membranes;

Contains its own genome, a circular molecule of DNA (35,000 base pairs) which encodes

Clearly 30 proteins are not enough to accomplish so many functions so the apicoplast has to import from the cytosol ~500 nuclear-encoded proteins.

The apicoplast is the product of an ancient endosymbiosis in which the eukaryotic ancestor engulfed a unicellular alga — probably a red alga — with a solitary chloroplast. Over time, the nucleus was lost (no residual nucleomorph) as well as many features of the chloroplast (including its ability to perform photosynthesis).